1. Field of the Invention
The present invention relates to a wideband optical fiber amplifier having an amplification bandwidth at 1480-1520 nm which is a low loss region of an optical fiber.
2. Description of the Related Art
In conjunction with the spread of the Internet, etc., the communication capacity has been increasing rapidly so that the use of a communication system in the WDM (Wavelength Division Multiplexing) scheme as a large capacity optical communication system is becoming popular. In this WDM system, it is indispensable to use an EDFA (Erbium-Doped Fiber Amplifier) as a repeater, and the WDM system with the EDFA having an amplification bandwidth of 1.53-1.60 xcexcm has been available.
However, in order to realize a larger capacity for the communication facility, there is a need to enlarge the amplification bandwidth of the optical fiber amplifier, and there are great demands for a development of an optical fiber amplifier capable of covering a low loss region (1.45-1.65 xcexcm) of a silica fiber.
To this end, an optical fiber amplifier adapted to an S-band (1480-1520 nm) in which the silica fiber has low loss and low dispersion equivalent to the already available C-band (1530-1560 nm) has been developed. There are currently three types of such an optical fiber amplifier in the S-band.
The first one is a Raman optical fiber amplifier that utilizes the induced Raman scattering that occurs when a signal light is entered into a silica fiber in a state of having an intense pump light incident thereon (see J. Kani, et al., Electronics Letters, Vol. 34, No, 18, pp. 1745-1747, September 1998, for example).
The second one is a dual wavelength pumped TDFA (Thulium-Doped Fiber Amplifier) in which the population inversion is low and the amplification bandwidth is shifted to the long wavelength side by adding wavelengths with a high efficiency for pumping from the ground level to the amplification final energy level in the TDFA with 1000 nm band upconversion pumping which has the amplification bandwidth at Sxe2x88x92-band (1450-1480 nm) (see, T. Kasamatsu, et al., Optical Amplifiers and their Applications ""99. Optical Society of America Trends In Optics and Photonics Series Vol. 30, pp. 46-50, June 1999, for example).
FIG. 1A shows the energy levels of Tm and the amplification state of the dual wavelength pumped TDFA. For the amplification of the S-band, the stimulated emission from 3H4 to 3SF4. In the case of the dual wavelength pumping, the signal light is pumped from the ground level 3H6 to the amplification final energy level 3F4 by the 1560 nm pump light, and then pumped from the amplification final energy level 3F4 to the amplification initial energy level 3F2 by the 1000 nm pump light. By controlling the powers of the pump lights in two wavelengths so as to control the number of Tm ions (Tm3+) at each level, a low population inversion state is formed and the amplification bandwidth of the TDFA in the S+-band is shifted to the S-band.
Moreover, as shown in FIG. 1B, when the pumping wavelength from the amplification final energy level 2F4 to the amplification initial energy level 3F2 is changed from the 1000 nm band to the 1400 nm band which has the higher pumping efficiency, it is possible to realize the high efficiency S-band optical fiber amplifier (see, T. Kasamatsu, et al., Electronics Letters, Vol. 36, No. 19, pp. 1607-1609, September 2000, for example).
The third one is a high Tm3+ concentration TDFA in which the low population inversion is formed by the cross relaxation among Tm3xe2x88x92 generated In the pumping state and the amplification bandwidth is shifted to the S-band on the long wavelength region, by adding Tm which is the additive ions to the optical fiber core which is the amplification medium at a high Tm3+ concentration, in the 1000 nm band upconversion pumping TDFA (see, S. Aozasa, et al., Electronics Letters, Vol. 36, No. 5, pp. 418-419, March 2000, for example).
FIG. 2 shows the energy levels of Tm and the amplification state of the high Tm3 concentration TDFA. The signal light is pumped once from the ground level 3H6 to the amplification final energy level 3F4 by the 1000 nm pump light, and then pumped further from the amplification final energy level 3F4 to the amplification initial energy level 3F2 by the pump light of the same wavelength.
Now, in the low Tm3+ concentration TDFA, the high population inversion is formed because the absorption of Tm3xe2x88x92 with respect to the pump light at a time of pumping from the amplification final energy level 3F4 to the amplification initial energy level 3F3 is higher than the absorption of Tm3+ with respect to the pump light at a time of pumping from the ground level 3H3 to the amplification final energy level 3F4.
Such a low Tm3+ concentration TDFA has the amplification bandwidth mostly in the S+-band described above as a result of the high population inversion state, and the amplification operation can be realized even in the S-band although it is deviated from the peak wavelength of the gain spectrum. The amplification efficiency in the S-band by this low Tm3+ concentration TDFA is less than or equal to that of the high Tm3+ concentration TDFA.
In contrast, in the high Tm3+ concentration TDFA, the interaction among Tm3+ occurs so that, as shown in FIG. 2, Tm3+ pumped to the amplification initial energy level 3F2 is relaxed to the amplification final energy level 3F4 by causing the energy transfer to the neighboring Tm3xe2x88x92 at the ground level 3H6, while Tm3+ that received the energy is pumped to the amplification final energy level 3F4. As a result, the number of Tm3+ pumped to the amplification final energy level 3F4 is increased so that the low population inversion is formed and the gain shirt occurs.
However, in the high Tm3+ concentration TDFA described above, the laser diode (LD) for emitting the 1000 nm wavelength band to be used for the pump light has not been developed yet, so that its practical realization has been difficult because it is difficult to realize a low cost and a compact size, and the conversion efficiency is not very good (approximately 5%).
Also, even in the low Tm3+ concentration TDFA, the laser diode (LD) for emitting the 1000 nm wavelength band to be used for the pump light has not been developed yet, so that its practical realization has been difficult because it is difficult to realize a low cost and a compact size, and the conversion efficiency in the S-band is less than or equal to the high Tm3, concentration TDFA.
It is therefore an object of the present invention to provide an optical fiber amplifier with a high conversion efficiency which is capable of using the pump light in the wavelength band that can be emitted by the laser diode.
According to one aspect of the present invention there is provided an optical fiber amplifier, comprising: an amplification optical fiber containing thulium at least in a core, to which a signal light is to be entered; and a pump light input unit configured to enter at least one pump light with a wavelength in a range of 1320-1520 nm into the amplification optical amplifier.
According to another aspect of the present invention there is provided an optical fiber amplifier, comprising: a plurality of amplification optical fibers each containing thulium at least in a core, to which a signal light is to be entered, the plurality of amplification optical fibers being connected in series or in parallel; and a plurality of pump light input units each configured to enter at least one pump light with a wavelength in a range of 1320-1520 nm into a respective one of the amplification optical amplifiers.
Other features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings.